Method of stabilizing reflectors



Patented June 23, 1936 UNITED STATES PATENT OFFICE METHOD or STABILIZING REFLECTORS Martin Tosterud, Arnold, Pa., assignor to Alumi num Company of America, Pittsburgh, Pee, a corporation of Pennsylvania No Drawing. Application December 27, 1932, Serial No. 649,111

3 Claims. (01. 148-6) The invention relates to reflecting surfaces of with the invention, treated with a hot solution aluminum and to methods by which the reflectof chromate. The exact physical method of ing power of such surfaces may be substantially bringing the solution to the metal surface is not stabilized. important; suflice it to say that any method of The metal aluminum (or its aluminum-base application by means of which the bright alumi- 6 alloys) when provided with a polished surface, is num surface is covered with the hot solution of a very efflcient reflector of radiant energy. For chromate will be adequate. Dipping, spraying, visible light, such a surface has a reflectivity of brushing and the like are all possible processes, about 70 to 80 per cent, more or less. For infrabut in normal practice it is usually physically red or heat radiation, the reflectivity of polished and economically convenient to simply immerse 10 aluminum surfaces is about 90 per cent or more. the metal in the hot solution. Another method The bright metal likewise has a very low emis whichmay prove convenient is that of spraying sivity. The metal is therefore used for reflecting or otherwise placing a thin layer of chromate surfaces and finds wide usage-in lighting reflecsolution on the metal and then placing the tors. Usuallyinthe form of foil, aluminum finds wetted metal in a steam chamber or an oven 15 application in insulating structure's and is used where the high temperatures will quickly develop as such in many places, including refrigerators, a high temperature at the interface between the ships, trucks. In such usages the polished alumimetal and the chromate solution. The main varinum surfaces of low emissivity bound dead air ables to be considered are those of time of con- 29 spaces. tact with the solution, concentration of chrom- Reflecting aluminum surfaces are, in their wide ate in the solution, and temperature of the soluand varied uses, .placed in many environments. tion. Of these variables, e perature is of ma- While under many conditions of service, the rejor importance. The concentration of chromate f fleeting power of the aluminum surface is stable in solution and the time of treatment are to and remains constant, or substantially so, for some degree dependent, and generally alow con- 25 long periods of time, there are other adverse centration of chromate will require a longer time service conditions where the reflectivity is unof treatment. But in any case, a comparatively stable and tends to deteriorate gradually or even short treatment, for instance one of 5 to 60 rapidly to values which are, comparatively speakminutes, will sufflce. The temperature of the ing, very low. While the need for a treatment solution is more critical. Hot solutions are pref- 30 which would stabilize the reflectivity of polished r bly d, nd wh n the r icle is immersed aluminum surfaces has long been felt, such treatin such solutions the solutions are best held at ments as have heretofore been tried have been approximately 75 centigrade 'or more. The unsatisfactory for one reason or another.. boiling temperature is preferred since at this The method of the present invention has been temperature the best results are obtained and developed with the end in view of stabilizing the likewise it is a solution temperature which is metal reflecting surface and it consists, in brief, readily maintained constant. of treating the bright aluminum surface in a The effects obtained in the treatment of bright hot solution of a class of salts herein termed aluminum surfaces by the methods of this inven- 40 chromates. The chromates are of two general tion are of a major order. The initial reflectivity 40 typesthe true chromates, which are the more of the aluminum surface is substantially malneflicient, and the dichromates. Both serve the tained but the stabilizing effect of the treatment purpose of the present invention, provided, of is such that decreases in reflectivity of the order course, that they are appreciably soluble and of per cent, as sometimes occur under severe e5 do not attack the aluminum surface. The class conditions, are prevented for long periods of of salts comprehended by the term chromate time. is, therefore, inclusive of the appreciably solu- As an example of the efficiency of the method ble chromates and dichromates. Of this class of stabilizing the reflectivity of aluminum surthe alkali metal salts are preferable because of faces may be cited the following severe tests:-- 50 their ready availability, low cost, and high solu- Aluminum sheet, with a bright reflecting sur- 5 bility, and such salts will be specifically referred face derived from being finished on polished to in the description of the methods of this inrolls, was treated by the method of my invention. vention. One lot was boiled for 30 minutes in a 10 per cent The bright aluminum surface, the reflectivity solution of potassium dichromate. The second of which is to be stabilized, is, in accordance lot was boiled for 15 minutes in a 10 per cent solution of potassium chromate, and a third lot, without treatment, was included in the test. It is known that .wetting of reflecting aluminum surfaces, or condensation under high humidity conditions is a severe test of the durability of reflecting surfaces, and for the present test, samples of the metal above-described were immersed in tap water for a period of one and onehalf years, after which the samples were removed and their reflectivities measured. The original metal before treatment had a reflectivity of about 68 per cent. This reflectivity was not decreased by boiling in chromate solution. However, after one and one-half year's immersion in tap water,

, the untreated metal had its reflectivity decreased to 25 per cent, whereas the metal treated in dichromate solution had a reflectivity of 58 per cent, and the metal treated in chromate solution had a reflectivity of 63 per cent.

In another test, bright aluminum foil having a reflectivity of 80 per cent was treated for 10 minutes in a boiling 10 per cent solution of sodium chromate. The foil, after treatment, showed no loss in reflectivity. Samples of the treated and untreated fofl were exposed in an atmosphere of 100 per cent humidity at about 30 centigrade, and under conditions which caused the surface of the foil to is continually wet with condensedmoisture. After 100 hours exposure under these conditions, the sample of untreated foil had turned quite dark and had a reflectivity of only 12 per cent, whereas the sample of foil treated with the sodium chromate solution had a reflectivity of 79 per cent, showing substantially no loss in reflectivity.

I claim:

1. The method of stabilizing the reflecting power of aluminum surfaces consisting of heating said surfaces in contact with a solution of a substantial amount of chromate in water.

2.- The method of stabilizing the reflecting power of aluminum surfaces consisting of applying to said surface a solution of a substantial amount of chromate in water at a temperature of at least about 75 centigrade.

3. The method of stabilizing the reflecting 20 

